Both electromagnets and permanent magnets have been used to manipulate beams of charged particles. In traveling wave tubes, for example, magnets have been arranged around the channel through which the beam travels to focus the stream of electrons; that is, to reduce the tendency of the electrons to repel each other and spread out. Various configurations of permanent magnets (and pole pieces) have been tried in an attempt to increase the focusing effect while minimizing the weight and volume of the resulting device. In conventional traveling wave tubes, permanent magnets are typically arranged in a sequence of alternating magnetization, either parallel to, or anti-parallel to, the direction of the electron flow. The magnets (and pole pieces) are usually annular in shape and their axes are aligned with the path of the electron beam. Pole pieces, constructed of ferromagnetic material such as electrolytic iron, are often placed between the magnets and provide a path through which magnetic flux from the magnets may be directed into the working space along the axis of the traveling wave tube in order to influence the beam in the desired manner. The patent to Clarke, U.S. Pat. No. 4,731,598, issued Mar. 15, 1988, illustrates typical prior art, periodic permanent magnetic (PPM) structures.
One of the critical problems confronting those who develop magnetic structures used to contain or manipulate beams of charged particles has been how to more efficiently utilize the permanent magnet materials which make up the structure(s). Some specific problems: How to maximize the strength of the magnetic field along the path of the charged particle beam without increasing the mass of the magnetic structure; how to increase heat dissipation from the central cavity to the outside atmosphere; and how to gain access to the interior of the structure in a simple and efficient manner.